Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its sensitivity to octane-enhancing additives.
Industrial alkylation processes have historically used large volumes of liquid Bronsted acid catalysts such as hydrofluoric or sulfuric acid under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid.
Bronsted acid catalyzed isoparaffin:olefin alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Researchers in the field have addressed the safety and environmental concerns surrounding the use of Bronsted acids in various ways, including methods for containing and/or neutralizing acid clouds following accidental releases. See, for example, U.S. Pat. Nos. 4,938,935 and 4,985,220 to Audeh and Greco, as well as U.S. Pat. No. 4,938,936 to Yan.
The two-part article, "Modern Alkylation", by Lyle F. Albright, Oil and Gas Journal, Nov. 12 and 26, 1990, summarizes the state of the art in alkylation technology, and highlights problems associated with liquid Bronsted acid catalysts such as HF and H.sub.2 SO.sub.4, and notes safety and environmental concerns associated with using and storing substantial quantities of these acids, which concerns underscore the desirability of developing a commercially viable low acid inventory isoparaffin:olefin alkylation process.
Conventionally, hydrocarbon reactants are bubbled through a vessel (HF alkylation) or vigorously stirred with the acid to assure adequate contact between the acid and the hydrocarbon reactants (e.g., H.sub.2 SO.sub.4 alkylation in a Stratco brand horizontal impelled reactor/heat exchanger). Various techniques have been explored for improving contact between a Bronsted acid catalyst and hydrocarbon reactants. For example, U.S. Pat. No. 3,780,130 describes a gas-fog alkylation process, wherein a fog or mist of acid is allowed to react with the hydrocarbon. Although this prior art noted some improvement in the alkylation process, very high voltages (up to 4000 v) are required for the generation of acid fog making the process impractical on a commercial scale.
In the process described in U.S. Pat. No. 2,380,234, a small amount of solid is dispersed in the acid phase. The resultant system shows limited improvement in the alkylation efficiency as shown by a slight increase in the alkylate yield.
More recently, U.S. Pat. No. 4,783,567, teaches a process wherein the hydrocarbon feed is contacted with hydrofluoric acid in a reactor with a fixed bed of a solid packing. The reference reported minor improvements associated with the use of solid packings.
Researchers seeking to improve reaction temperature uniformity and reactant/catalyst contact efficiency in liquid acid-catalyzed alkylation processes have developed improved reactor configurations to achieve these objectives, as shown by the following references.
U.S. Pat. No. 3,456,033 to Borst, Jr. relates to an alkylation process wherein reactant hydrocarbons and acid catalyst flow through a plurality of heat exchange tubes while a gaseous heat exchange medium passes across the tubes to remove heat of reaction.
U.S Pat. Nos. 3,469,949 and 3,501,538 to Borst, Jr. disclose a process and apparatus for carrying out an alkylation reaction which jets the hydrocarbon reactants into an upwardly flowing stream of hydrogen fluoride catalyst to impart a spiral flow path to the hydrocarbon reactants.
U.S. Pat. No. 3,817,708 to Vernon teaches an apparatus for an alkylation process, which apparatus comprises a pair of interconnected u-tube heat exchangers. The disclosed configuration is said to increase the capacity of a conventional liquid acid alkylation process.
U.S. Pat. No. 3,914,110 to Anderson teaches an alkylation reaction cooler comprising a heat exchanger, a plurality of baffles and a plurality of spray nozzles assemblies within a horizontal elongated chamber. Alkylation reactants are sprayed into the acid catalyst which flows through the elongated horizontal chamber in a serpentine flow path.
U.S. Pat. Nos. 4,783,567 and 4,891,466 to Kocal disclose alkylation processes in a vertical fixed bed which is said to improve process efficiency.
Years of industrial experience have proven that liquid Bronsted acid alkylation catalysts such as HF and H.sub.2 SO.sub.4 can be handled safely. To assure continued compliance with increasingly stringent regulations governing the handling and storage of potentially hazardous materials, research efforts have now been directed toward reducing the required inventory of liquid acid catalyst in industrial isoparaffin:olefin alkylation units.